Thanks Tony that made a lot of sense and that was the best explanation so far actually.

"Why are certain throws illegal in MuayThai?
It is my understanding that there are two reasons certain throws are illegal. The first reason concern safety; some throws offer the possibility of lasting or permanent damage (cuts from an elbow heal relatively quickly). Other throws are excluded because they are considered to be techniques from other martial arts and not MuayThai. This last point is a contentious issue, some trainers in Thailand feel that the art of MuayThai is being eroded and more and more techniques are being taken away by the rules. For example, Kon Khun Yak Pa Nang is a traditional technique that involves a hip throw; MuayThai not judo. "

Safety concerns I can somewhat see as this is a ring sport and not a nice padded mat one. It also makes one wonder if muay thai is more and more defined by sports rules and is in fact losing much of it's identity because of it. A great post and it is very interesting.